where \(Q_{f}\)  is the charge of quarks, and \(\langle \bar{q}q\rangle \)  is obtained from Eq. (REF ).
One might suspect that \(\chi _{f}\)  and \(\langle \bar{q}\sigma ^{12}q\rangle /eB\)  are not completely unrelated.
Indeed, as Bali and the collaborators have discussed in Ref. [1]}, [2]},
\(\langle \bar{q}\sigma ^{12}q\rangle /eB\)  represents the contribution of the spin of quark flavor to
the total magnetic susceptibility (such total magnetic susceptibility is composed by spin-related
and orbital angular momentum-related contributions).
Here, we consider the spin-related quantity using the parameter \(\kappa _{2}\) .
Noting that, the sub-index \(f\)  in Eq. (REF )-Eq. (REF ) are the flavor of quark.
